I took bitcoin as an example but we can talk about cryptocurrency that  will solve the problem of scalability.
So are there any problem theoretically speaking substituting dollars with a cryptocurrency?


Theoretically no. But the problem i see if you are looking at a simple "replacement" is all this years of accumulation of fiat and printing of new bills to maintain price. How can you replace something that as no value with something that is scarse and theoretically non inflationary. I think we can see at least a change in our finantial and monetary system, if not a small revolution.